avcodec/h264: allow mixing idr and non idr slices with frame threading again

This combination exists in the wild
Fixes Ticket3131
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264.c

@@ -4879,7 +4879,9 @@ static int decode_nal_units(H264Context *h, const uint8_t *buf, int buf_size,
                 case NAL_IDR_SLICE:
                 case NAL_SLICE:
                     init_get_bits(&hx->gb, ptr, bit_length);
-                    if (!get_ue_golomb(&hx->gb) || !first_slice)
+                    if (!get_ue_golomb(&hx->gb) ||
+                        !first_slice ||
+                        first_slice != hx->nal_unit_type)
                         nals_needed = nal_index;
                     if (!first_slice)
                         first_slice = hx->nal_unit_type;
@@ -4927,7 +4929,7 @@ again:
 
             switch (hx->nal_unit_type) {
             case NAL_IDR_SLICE:
-                if (first_slice != NAL_IDR_SLICE) {
+                if (h->nal_unit_type != NAL_IDR_SLICE) {
                     av_log(h->avctx, AV_LOG_ERROR,
                            "Invalid mix of idr and non-idr slices\n");
                     ret = -1;